[Rachel's Introduction: A study of more than 300 people with Parkinson's disease found that sufferers were more than twice as likely to report heavy exposure to pesticides over their lifetime compared to family members without the disease.]

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By Alok Jha, science correspondent

Scientists have found further evidence of a link between Parkinson's disease and long-term exposure to pesticides.

A study of more than 300 people with the neurological disease -- which can affect movements such as walking, talking and writing -- found that sufferers were more than twice as likely to report heavy exposure to pesticides over their lifetime as family members without the disease.

Previous studies have pointed to a possible link between pesticide exposure and Parkinson's and public authorities are trying to work out whether these risks should be classed as significant. A &pound;906,000 project to study the links launched in 2006 by the Department of Environment, Food and Rural Affairs, for example, is due to report this summer.

Variations in several genes have been identified that contribute to the disease, but these defects are rare and only account for a small proportion of the incidence of the disease, which afflicts around 120,000 people in the UK. The majority of cases are thought to be a result of an interaction between genes and the environment.

Lifetime exposure

The new research, led by American scientists, looked at the lifetime pesticide exposure of 319 Parkinson's patients and more than 200 of their relatives without the disease. The results, published today in the journal BMC Neurology, showed that people with Parkinson's were 1.6 times as likely to report an exposure to pesticides in their lifetimes compared with the controls.

In addition, people with the Parkinson's were 2.4 times as likely as people without the disease to report heavy exposure to pesticides, classed as more than 215 days over a lifetime.

The strongest associations were between people with Parkinson's who had been exposed to herbicide and insecticide chemicals such as organochlorides and organophosphates. No links were found between Parkinson's disease and drinking well-water or living or working on a farm, two commonly used proxies for pesticide exposures.

"In this dataset, these tended to be people who used a lot of pesticides in their homes and in their hobbies," said William Scott of the University of Miami, who took part in the study. "There were not many people who routinely used pesticides for their occupation."

Though the evidence is growing, the researchers said that there was not enough biological evidence yet to conclude that Parkinson's was definitely caused by pesticide exposure. The biological mechanism linking the two is still unknown. The researchers added that future genetic studies of Parkinson's could consider the influence of pesticides, because exposure to these chemicals may trigger the disease in genetically predisposed people.

Key role

Kieran Breen, director of research at the Parkinson's Disease Society (PDS), said: "The association between pesticides and Parkinson's has been recognised for some time, and this study supports this link and strengthens the fact that pesticides play a key role."

The PDS has carried out a survey of more than 10,000 people with Parkinson's and preliminary results show that 9% had long-term pesticide or herbicide exposure, which is defined as exposure for more than a year.

"Of the 3,000 carers surveyed, most of whom were family members, less than 2% had had similar exposure," said Breen. "This demonstates that pesticides may be contributing to nerve cell death in some people with Parkinson's, but is unlikely to be the only cause."

Symptoms of the disease first tend to appear when a patient is older than 50, and can include tremors and muscle rigidity. The Parkinson's Disease Society estimates that around 10,000 new diagnoses of the disease are made ever year in the UK.